1. Field of the Invention
The present invention relates to films for use as antireflective coatings and hardmasks for lithographic structures. More particularly, the present invention relates to optically tunable, water and/or aqueous base soluble materials for use as antireflective coatings, hardmasks, and combined antireflective coating/hardmasks.
2. Description of the Prior Art
The need to remain cost and performance competitive in the production of semiconductor devices has driven the industry to a continuing increase in device density with a concomitant decrease in device geometry. To facilitate the shrinking device dimensions, new lithographic materials, processes and tools are being considered. Currently, 248 nm lithography is being pursued to print sub-200 nm features. To do this, tools with higher numerical aperture (NA) are emerging. The higher NA allows for improved resolution but reduces the depth of focus of aerial images projected onto the resist. Because of the reduced depth of focus, a thinner resist will be required. As the thickness of the resist is decreased, the resist becomes less effective as a mask for subsequent dry etch image transfer to the underlying substrate, i.e. most if not all of the resist is etched away during the subsequent pattern transfer process. Without significant improvement in the etch selectivity exhibited by current single layer resists (SLR), these systems cannot provide the necessary lithographic and etch properties for high-resolution lithography.
Typical resist structures consist of a resist on top of an antireflective coating (ARC). The resist is exposed and developed and the image is then transferred through the ARC and then through the underlying layers, such as oxide, nitride or silicon layers. Typical resist thickness is on the order of 5000 A for the current state-of-the-art lithography process. During the ARC open, significant resist is lost as the etch selectivity between the resist and ARC is at best 1:1. As minimum features continue to decrease, it is desirable to thin the resist to attain the high resolution as well as improved process window (exposure and focus latitude). However, thinning the resist below 5000 A poses etch problems. There may be insufficient resist to function as an etch mask for subsequent transfer etch into underlying layer. Compounding this problem is the fact that significant resist loss also occurs during the ARC open.
Another problem with single layer resist systems is critical dimension (CD) control. Substrate reflections at ultraviolet (UV) and deep ultraviolet (DUV) wavelengths are notorious for producing standing wave effects and resist notching, which severely limit CD control of single layer resists. Notching results from substrate topography and non-uniform substrate reflectivity, which causes local variations in exposure energy on the resist. Standing waves are thin film interference (TFI) or periodic variations of light intensity through the resist thickness. These light variations are introduced because planarization of the resist presents different thickness through the underlying topography. Thin film interference plays a dominant role in CD control of single layer photoresist processes, causing large changes in the effective exposure dose due to a tiny change in optical phase. Thin film interference effects are described in “Optimization of optical properties of resist processes” (T. Brunner, SPIE Proceedings Vol. 1466, p. 297, 1991), the teaching of which is incorporated herein by reference.
Bottom anti-reflective coatings or BARCs have been used with single layer resists to reduce thin film interference. However, these thin absorbing BARCs have fundamental limitations. These materials are generally spin applied. The thickness of the BARC and the resist cannot be controlled to the accuracy required to operate at the target thickness to achieve minimum reflectance. The resist thickness can also vary due to existing topography. Thin underlying films such as silicon nitride or silicon oxide tend to exhibit some thickness non-uniformity after deposition. The thin BARC will generally not effectively planarize these thin underlying films. Thus, as a result there will be a variation in exposure energy into the resist. Current trends to reduce topography via chemical/mechanical polishing still leave significant variations in film thickness over topography.
To overcome some of the limitations of single layer resists, thin film imaging techniques have been developed including bilayer resists, trilayer resist systems and top surface imaging (TSI). In a bilayer structure, a thin resist containing Si functionality for etch resistance is coated on top of a thick polymer layer with suitable absorption at the exposing wavelength to act as a BARC and suitable etch resistance for substrate etch. Because of the thick resist/underlayer stack, this technique offers tremendous advantage for etch transfer. However, incorporation of Si moieties into the imaging resist structure is very challenging and can result in limited resolution and low performance of such resist systems. All of these thin film imaging techniques are more complex and costly than current single layer resist processes.
The importance of hardmask technology is becoming increasingly evident as the demand for high-resolution imaging dictates the use of ever-thinner resist films. An appropriately designed etch resistant hardmask used in conjunction with a thin resist can provide the combined lithographic and etch performance needed for sub-100 nm device fabrication. Plasma-enhanced chemical vapor deposition (PECVD) prepared materials that perform both as an antireflective coating (ARC) and hardmask offer several advantages over organic bottom antireflective coatings (BARC) currently used for manufacturing of logic and memory chips. These benefits include excellent tunability of the material's optical properties, which allows superior substrate reflectivity control, high etch selectivity to resist, exceeding 2:1 due to the significant difference in chemical composition between resist and PECVD deposited material. In addition, PECVD deposited materials are highly crosslinked covalent networks which are considerably denser compared to organic materials thus can serve as an effective hardmask etch barrier during the plasma etching of dielectric stacks. In contrast, organic BARCs have poor optical tunability, which means that their index of refraction, n, and the extinction coefficient, k, cannot be finely tuned to match resist and substrate optical properties. Additionally, organic BARCs have chemical composition very similar to resist materials which results in poor etch selectivity of about or less than 1:1 between resist and BARC to resist. Thus, about 100 nm of the resist is consumed during ARC open, with high-resolution imaging, this becomes a fundamental process limitation.
Recently, considerable interest has been focused on PECVD prepared ARC/hardmask materials as they offer tremendous leverage for extending optical lithography to sub 100 nm resolution. Such materials are described in U.S. Pat. Nos. 6,316,167 and 6,514,667. Typically, ARC/hardmask materials must be removed after the lithographic patterning is complete as their presence in the final device structure can adversely affect the device performance. The PECVD ARC/hardmask materials described in these two patents are highly cross-linked covalent networks which are significantly dense compared to organic polymer films and hence difficult to remove with conventional wet and dry strip processes without damaging the layers underneath. This significantly limits their use in semiconductor processing.
Depending on the particular integration structure, PECVD ARC/hardmask materials must be removed from the structure selective to one or more substrate materials. The substrate can be a dielectric material e.g. silicon oxide or silicon nitride, low dielectric constant materials e.g. SiCOH and ultra-low dielectric constant materials e.g. porous SiCOH and/or a semiconductor such as polysilicon and/or a metal e.g. copper, aluminum. Improved ARC/hardmask materials that can be selectively removed are needed.
It is desirable to develop a thin resist process which provides excellent lithographic performance and provides appropriate etch resistance for effective pattern transfer into the underlying substrate. In order to do this, improved ARC/hardmask materials are needed which provide better etch selectivity to resist than current organic BARCs. The ARC/hardmask material needs to (1) have appropriate optical properties to function as a suitable ARC at appropriate wavelength, (2) provide good etch selectivity to resist (greater than 1:1), and (3) does not interact with the resist inducing residue, footing, undercut thereby limiting the overall lithographic performance of the resist. It is also desirable that the ARC/hardmask material also function as a suitable hard mask material for the underlayer etch. Germanium based ARC/hardmask materials prepared by PECVD processes and spin coating processes whose properties are tailored to allow selective removal of these materials from the structure in water or aqueous base solutions are disclosed herein.